The physiology of pituary-gonadal feedback relationships will be studied in rodents (mostly rats) and squirrel monkeys using radioimmunoassay of steroids and gonadotropins, chronic intravascular catheterization and constant release capsules for steroid administration. Quantitative and qualitative aspects of the gonadotropin - testicular relationship will be studied, particularly as regards negative feedback effects of testosterone. The origins of changes in pituary sensitivity to LH releasing factor will be assessed, with particular reference to the hypothesis that such changes may be dependent upon total stores of pituary LH or qualitative changes in the LH molecule. Attempts will be made to establish definitively whether the onset of male puberty is related to a real change in the threshold of sensitivity to gonadal steroid feedback. Circulating steroid levels will be monitored in the last mentioned experiments to allow for possible changes in distribution and/or clearance of hormone with age. Attempts will also be made to locate the site of feedback action of gonadal steroids under relatively normal physiological conditions, using systemic and intracranial administration of steroids combined with repeated testing of pituitary sensitivity to LH releasing factor. A hypothesis of direct neural regulation of testosterone secretion will be explored. BIBLIOGRAPHIC REFERENCES: Sodersten, P., G. Gray, D. A. Damassa, E. R. Smith, and J. M. Davidson. Effects of a non-steroidal antiandrogen on sexual behavior and pituitary-gonadal function in the male rat. Endocrinology, 1974, December issue. Davidson, J. M., D. S. Damassa, E. R. Smith, and C. Cheung. Feedback control of gonadotropin secretion in the male rat. In: Regulatory mecahnisms of male Reproductive Physiology. C. Spilman (ed.), Elsevier, (in press).